Telephone repeater



staresinercia r erica- LVA B. orfanner'snoonrn, New Yoan, AssiGNonfro AMERICAN TnLErHoNE f Ann'rnLneaarI-I COMPANY, a CORPORATION, or NEW YORK.

irnLErHoNn, Barrieren. I j l' Application sied July 31, 191e. serial No. 31e-.547.-

This inventionvre'lates to vtelephone, repeeters; more particularly it concernsiarrangements of circuits land apparatus for preventing repeater singing.

The singing of a repeater,- 'as well understood, is caused vby imperfect conjugacy between the input and output circuitsofthe repeater, aportion of the output current entering the input circuit and thusocausing a local circulation 'of energy. Perfect lcon- 4jugacy can be obtained only ifthe 'transmission line with. which t. e repeater is 'associated is -in perfect badante at allfrequencies of current. Experience has-shown that, al-

though it is a comparatively simple matter to balance a line for frequenciesv within the useful telephone range it is a matter of eX- treme ditiiculty 'and high expense evento approximate line balance for certain frequencies above this range, since the inn-- pedance of trans-mission lines' for such fre quencies cannot closely simulated by means of simpleI networks. A repeater. is therefore apt to singiat the high frequencies and itis customary to associate with thefrepeater a filter which serves to attenuate and approximately extinguish currents abov e i say 2200 cycles, lwhile it vtransmits with4 practically negligible'loss frequencies below this range. In the repeatersnow in use this filter is associated with the input circuit where it suppresses the high frequencies before they reach the amplifying device. This arrangement is open to certain objections explained hereinafter and thejessence of the present invention is to provide a4 new and improved arrangement of the repeater apparatus in which. arrangement a filter is associated with the amplifying device in its output circuit. j

The above and other-,features of this invention will be readiy understood from the followin d scription 'considered' in` connection wit v t e accompanying drawing, in --whch Figure l is a diagrammatic Aview showing an arrangement lot apparatus and circuits embodying this invention associatedwith "a well known .type of one-element. repeater, Fig. 2l being a diagrammaticjview similar to Fig. l, showing-the arrangement of this invention associated with a well known type of two-element repeater.

In Fig. l, yreference character L designates a telephone iine across which is bridged a, potentiometer 2 having associated therewith in variable" relationship the input transformer 8 of an amplifying'device which is f here shown. as an 4electron tube 4. This tube is provided with the usual filamenQ-` plate and grid, supplied with current and vpotential .by the batteries A, B andyC,

respectively.. The output circuit' of 4the tube is led through a filter F to the output transformer l0, the secondary wind!v ings of which transformer are inserted in series with the line conductors and arranged symmetrically with respect to' the points at which the potentiometer 2 is connected tothe line. The sections oftheline L extending to the left and to the right'of the repeater are balanced for currents of voice frequencies in order to prevent singing of the repeaters' at these frequencies.` .Hal-

ance shouldalso exist between the twov conil 4ductors of each section of the line in order to prevent noise 'due to extraneous disurbances. The parts 'of the repeater conductively associated with the line are therefore., balanced, each of the variable taps of the 1 potentiometer being placed-.at an equal distance from the' adjacent terminal-H.

'The manner in lwhich the repeater l'operates is briefly as follows: Part of the current in. line L is shunted bythe potentiometer 2 and enters ,transformer 3, by means of which it is impressed on the electron tube. This device ampliiies Ithe'current and causes it to flow throughilter'F into transformer 10,

whereby it is returned to the linev L. The i filter suppresses currents of aj frequency higher. than 2200 cycles, and for frequencies belowfthis range substantiallyvno potential difference between the herminals'of the ypo-,fv tentiometer is produced by the electro-r`nonr tive force impressed on the line conductors by transformer 1Q, because .of .the balance of the line. Singing ofthe repeater atthe higher frequencies is. thus prevented by the filter, whereas'at the lower frequencies is effectively precluded, when the tube 1s operating -with Anormal amplification, bythe line.

balance., Where desired,vthe vtiltery may be arranged to' attenuate also currents of low frequency, permitting only such currents to pass as are necessary for the transmission of intelligible speech.` f The range of frequencies at which ak filter is effective is determined by 'th'e impedances of its elements, vand reference may be-had to the patent of G. A. Campbell, 1,227,114, May

:uml

l of the circuit.

22, 1917, for electrical receiving, translating or repeating circuit, for a description. of the manner in which the said' impedance values may be determined. ln the said patcnt and in fact in present practice, the'filter is in conductive connection with vthe line wires. To reserve the balance of thelinc, cach series impedance of the ilter conduc tively connected in one Wire must be balanced by a like impedance connected in the other. Each series inductance coil of the ilter is therefore in practice provided with two windings, one in cach and for pericct balance these windings must be alike in every respect.

The ilter of the present "arrangement isv associated with the output circuit o the electron tube. This circuit has no conductive connection with the line and need therefore not be balanced, The inductance coils oi.' other impedance of the filter serially connected into one conductor of the circuit need therefore not be balanced by like coils or otlierimpedanccs in the other conductor Moreover,- since for any cutoi frequency the iinpedances of the elements 'of a filter are determined by the impedance of the circuit into which the filter is con nected, itis readily understood that whenv a filter is conductively associated 'uiith a transmission line its impedances must be calculated relative to the impedance of the line, a filter ot different impedance relations beingr necessary for each diii'crentrline. ln the present arrangement, however, the impedance oit the filter is determined by the impedance of the output circuit of the amplifying device, which impedance ,is readily determinable and does not vary with each line: The adjustment of the filter .im-

pedance to the liuc is readily made' at the transformer l0. Because of the high impedance of the output circuit of the electron tube the condensers required for the filter are quite small and inexpensive compared to the condensersrcquired in the arrangement heretofore used.

Moreover, under the conditions usually eX- isting in an electron tube telephone` repeater circuitl the grid is substantially an insulated charged body except for its connection to the external circuit. The impedance of the input orgrid circuit of the electron tube is therefore extremely high, being principally the reactance due tothe electrostatic capacity of the grid to its surroundings. Furtlien more, to obtain an adequate output oiienergy from the electron 'tube a voltage must be impressed upon its grid which is high in comparison 'with those usually lexisting in telephone circuits,

tion ratio. This high transiorinationratio results in practical difficulties of transformer dosi transformer of reasonable size line conductor,

lassociating the this invention associated with a in this arrangement rhibitiup; these currents consequently the inputv` transformer 3 must have a large transiter-mater as the arrangement ci" Fig.

ncoasie and costhaving a suciently high self-in-v ductance 'for satisfactory performance at low frequency, has so much capacity in its windings as to be unsatisfactory athigh in quencies. The best available compromise design. has lower sel'-in'ductance in proportion to the impedance with which it Works lthan is the case 'with other transformers in the, repeater circuit, and in addition the ef feats of capacity are great enough to be injurious, The internal capacity of the input transformer together with the capacity of the grid and connecting Wires causes imn pedance effects at high frequencies and the relatively low lself inductance causes infr pcdance effects at low frequencies that are all unfavorable to the correct functioning of the filter when the same is connected, as heretofore., into the input circuit of the tube. {ilter With'the output cirn cuit removes it from theiniuencc of the input transformer thus improving its be liavior and in turnl improving; the charac teristics of the whole repeater set. In addition, due to its relatively low inductzmce, the efficiency of the input transformer is reduced at low frequencies by any impedance in series between it and the line. 'Acsociating the filter with the output circuit of the electron tube thus improves the behavior of the input impedance. l

*',lhis invention therefore provides 'for an airangement of the repeater circuits which results in archeap'er and simpler filter which has no tendency to unbalance .the line. It ano improves thevbehavior of both the input transformer and the filter by separating tli'em :from each other, A"The circuit arrangement of Fig., 2 shows repeater of thefainiliar two-element type. The line L is cnt at the place Where theirepcater is installed, both portions ot the line being balanced by networks N. An amplifying device, herein shown as an electron tube 4, is provided for each portion of thelne and associated therewith by a potcntioinetcr and a transformer .18, the output circuit of each tube being, associated with fthe'- portion of the line opposite. thereto by means oi an output transformer 10. l. A lter F is inserted into the outputleircuit of each tube, similarly as in Fig. il, and, prevents high frequency disturbances originating in one portion of the line from beingtransmitted to the other portion thereof, thus prol from causiiuc,r singing of tliefrep'eatcrs, When imbalance exists be tiveenftlie lines andthe networks, similarly as f described f f hereinbefore., The arrangement'of Fig. 2 has the same advantages relative tosimplicity yand clieapncss oit the il- Although-onlytwo forms and arrangetransformer by removing some oit the series lin-e L. Adjustment 'of the line, inductive 4means @coasts ments of apparatus and circuits embodying this invention arev shown and described herein, it is to be understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of the invention.

In both Fig. 1 and Fig. 2 it will be noticed that the potentiometer 2 is provided two adjustable contacts and that these are shown substantially at equal distances from the ends of the-Winding. Thus it Wild be seen that the potentiometer 2 stands in balanced relation to the two sides of the of the potentiometer may be made by moving the equal amounts oppositely so as to the balanced relation line L.

What is claimed is preserve with respect to. the

1. In combination, a Vtrimsmission line having its two sides in balanced relation, a vacuum tube amplifying device, a circuit inductively associating one side of said device with said line, and a wave iilter in said circuit, said 'Wave ilter'co'mprisin an impedance device in unbalanced re ation to' .the two sides of the is comprised.

2, In combination, a transmission iine liaving its two `sides in balanced relation, a vacuum tube repeater, a circuit inductiveiy associating one side of said repeater with said line, said circuit having its two sides in balanced relation, another circuit `assaid circuit in which it isociating the other side of said repeater with said line and having its two sides in un: balanced relation, and a wave filter 1n said last mentioned c1rcu1t. i

In combination, a transmission line of such length as to be subject to considerable `attenuation and external interference, a repeuter therefor comprising an electron tube am lifying device having an output circuitl of igli impedance relative to the impedance for associatin said circuit with said line and a filter in sai with and having certain elements in unbalanced` relation to the sides of said circuit. 1

5. In combination, a repeater, a transmission lineand a balanced potentiometer connecting said line and said repeater.

6, in combination, a transmission line hav two taps iii/ing its two sides in balanced relation and. a

repeater comprising an amplifying element, said repeater also comprising apparatus con- .nected with the line by a pair or conductors, said apparatus being in unbalanced relation to said pair of conductors ,and said apparatus being. placed on the output side of tbe amplifying element of the repeaters f 7 In combination, a transmission line, a r

repeater comprising apparatus in unbalanced. relation to the two sides of the line, said apparatus being placed on the output side of tbe repeater, and. a balanced potentiometer connecting the repeater with the 'tine on tbe input side.

8. In combination, a transmission li ne having its two sides in balanced relation and a repeater comprising a one-way amplifying element, said element having itsinput side connected with the line by means of a balanced potentiometer.

9. In combination, a repeater and a transmission line,- said repeater comprisingv a three-electrode vacuum tube and a baianced potentiometer, the said vacuum tube having its input side connected through said poten-- tiometer with said line.

in testimony whereof, i have signed m name to tliisuspeciication tlis 29th day of July 19,19. 

